Intelligent Roadway Information System
Travel Time Estimation
A route consists of one or more corridor trips. Each corridor trip has an
origin and a destination, which are
r_nodes associated with a single
freeway corridor. For the first corridor trip, the origin is the DMS location.
A route can only branch if an
r_node with exit node type is matched with
r_node. In order to match, the roadway / direction for the
r_node must equal the cross street / direction for the entrance, and
Two system attributes control route pathfinding.
route_max_legs is the
maximum number of corridors to branch.
route_max_miles is the maximum total
To deal with sampling problems at low speeds, a smoothing procedure is used. For each station, running average and running minimum speeds are calculated from all valid detectors in the station. If a speed higher than the speed limit is recorded, the speed limit is used instead.
The sample interval used increases as the speed becomes lower:
- Normally 2 minutes
- Extend to 3 minutes if < 25 mph
- Extend to 4 minutes if < 20 mph
- Extend to 5 minutes if < 15 mph
Route Travel Time
The distance between each consecutive valid station is divided into 3 links of equal length. If any link is longer than 0.6 miles, a travel time estimate would not be reliable, so the process is aborted. The links which are adjacent to a station are assigned speed from that station. “Middle” links are assigned an average of the immediate upstream and downstream station speeds.
Normally, the running average speed is used. If the link is less than 1 mile from the corridor destination, the running minimum is used instead. Travel time for each link is estimated by dividing its length by the assigned speed. If the route branches, each branch adds a 1 minute turning delay.
Travel Time Limit
To prevent unreasonable messages from being displayed due to data errors, a
limit is calculated. The total route length is divided by the value of the
travel_time_min_mph system attribute. The result, rounded up the to next 5
minutes, is the travel time limit.
If the calculated travel time is over the travel time limit, the message will be
over limit. If the over limit mode is
blank, no message will be shown.
Otherwise, the tag will be replaced with the travel time limit, with the over
limit text either prepended or appended.
Travel Time Action Tag
The estimated travel time (in minutes) can be displayed in DMS messages using
DMS actions. A
] action tag in the quick message will be
replaced with the appropriate value. It has the following format:
dest: Destination station ID
mode: Over limit mode (
blank: do not display travel time when over limit
prepend: prepend over limit text before travel time
append: append over limit text after travel time
over: Over limit text (
NOTE: multiple destinations can be included on the same message by specifying
FREEWAY TIME TO[nl][jl2]I-94[jl4][ttS100,prepend,OVER ] MIN
TIME TO[nl][jl2]I-35W[jl4][ttS200,append,+] MIN